Compiling projects with dependencies using GHC HEAD

Alexis King lexi.lambda at
Tue Oct 1 23:45:05 UTC 2019

> On Oct 1, 2019, at 18:19, Ben Gamari <ben at> wrote:
> Your question is a very good one which, until quite recently, there was
> not a particularly good answer to. However, in the past months we have
> been working on infrastructure (in the form of head.hackage [0]) to make
> GHC more testable in pre-release form.
> I have a draft blog post explaining the currently state-of-play
> here [1,2]. Unfortunately between ICFP, release things, vacation, and bug
> fixing I've lacked the time to finish these off (the tutorial in particular).

Hi Ben,

Many thanks for your response—this infrastructure looks incredibly helpful! I’ll give it a try. One question I do have just from giving it a look involves the cabal.constraints file available here: <>

A number of packages appear to be listed twice, and some of them (but not all of them) are pinned to different versions. Is that intentional? Or is it just a part of still being a bit of a work in progress?

In any case, this gets me quite a bit further, and though I’m not sure yet if my project will actually build, I can now at least construct a valid build plan. Thanks again!

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